Typical control circuits of the above-mentioned type are used, for example, for activating piezoelectric elements which act on a valve needle of a fuel injector, for example, in order to cause fuel injection into a combustion chamber of the internal combustion engine.
In order to achieve a stroke of the piezoelectric element which is sufficient for activating the valve needle, in conventional systems a control voltage of 160 V or more must be applied, which is typically clocked, in order to achieve controlled opening of the fuel injector and avoid damage to the ceramic of the piezoelectric element. In addition, an inductor is frequently provided in control circuits of this type, which limits the current in the piezoelectric element.
Because of the clocking of the control voltage, a triangular current curve is produced, and during the changes of the control current over time, voltage jumps also occur in supply line inductances of the piezoelectric elements, for example. The high-frequency voltage components accompanying the voltage jumps cause capacitive interference currents in particular, which flow via a housing of the fuel injector into the engine block, for example, and from there via a vehicle body connection back into the control unit, for example.
In this instance, the very low-resistance connection between the engine block, the vehicle body, and the control unit is particularly disadvantageous, due to which, together with the parasitic capacitances and possibly parasitic inductances making the interference currents possible, a high-quality resonance circuit is formed. The resonance frequency of the resonance circuit is determined by the magnitude of the parasitic capacitances and of the inductance of the return line via the engine block and the vehicle body connection. Typical capacitance values for the parasitic capacitances between the connecting lines of the piezoelectric elements and a housing of the fuel injector are, for example, approximately 500 pF to 1 nF, these values being able to vary depending on the arrangement of the piezoelectric elements or the fuel injectors.
The electromagnetic oscillations arising in the resonance circuit result in significant interference, for example, in radio receiver systems of a motor vehicle containing the control circuit or also in further electronic systems. Permissible limiting values for electromagnetic emissions of this type in the engine compartment are often exceeded.